In imaging devices for use in mobile phones and digital cameras, optical lenses for imaging are mounted. As materials for forming the optical lenses, optical glasses and transparent plastic materials have been generally used. In particular, with recent increase in the diffusion of the mobile phones fitted with an imaging function, it has been needed to produce the lenses more inexpensively and hence the transparent plastic materials having optical transparency have been required. Furthermore, with the recent trend of high resolution and thinning in the optical lenses for imaging, the materials having a higher refractive index (refractive index of 1.55 or more) have been needed.
As a method for producing optical components such as optical lenses using the transparent plastic materials, for example, there is mentioned a method including pressing a predetermined shaping mold against the transparent plastic material (resin composition) to fill the transparent plastic material into the mold, followed by releasing the material from the mold. That is, by transfer molding with the mold, an optical component having a specific micropattern or an odd shape can be obtained (for example, see Patent Document 1).
Depending on physical properties of the transparent plastic materials, from the viewpoint of dimensional stability, the following two types of curing methods are employed. Specifically, there are mentioned (1) a method including hot-melting a thermoplastic resin that is a transparent plastic material, then pressing a shaping mold against it, and cooling it to obtain a molded article (optical component) having a specific shape; and (2) a method including pressing the shaping mold against a photocurable resin composition as the transparent plastic material, and then irradiating it with light through the shaping mold or a substrate to obtain a molded article (optical component) having a specific shape.
In general, these two types of methods are selected depending on the required heat resistance temperature; and for example, in the field not requiring heat resistance, the method (1) of using a highly transparent thermoplastic resin such as polymethyl methacrylate (PMMA) or polycarbonate is widely employed. However, recently, for the purpose of more inexpensive and large-scale production, it is strongly desired to mount the imaging device itself on a printed board by reflow soldering. In that case, since the device is exposed to a high-temperature environment of softening temperature or higher at the mounting step, a problem of generation of softening and dimensional deformation remains in the use of the aforementioned thermoplastic resin.
From such a background, in the field of requiring heat resistance for reflow soldering or autoclaving etc., application of a photocurable resin including an epoxy resin as the main ingredient has been investigated and is being put into practical use. However, in the shaping method of optical components using the photocurable resin composition, at the release from the mold after curing by photoirradiation, there is a possibility that cohesive failure from the resin inside may take place owing to insufficient curing of the resin, whereby a defect may be generated in the molded article. Therefore, it is required to obtain a molded article having no defect with suppressing cohesive failure by improving the curability of the resin. Moreover, from the viewpoint of productivity, it is also required to be a material capable of attaining curing for a short period of time.
As a method for improving the curability of the photocurable resin composition, addition of an oxetane compound is generally known. In particular, an oxetane material having a primary alcohol (for example, 3-ethyl-3-hydroxymethyloxetane or the like) is known to exhibit a high effect of rapid curability because the hydroxyl group plays a role as a chain transfer agent in the photocuring reaction (for example, see Patent Document 2).
Patent Document 1: Japanese Patent No. 3926380
Patent Document 2: JP-A-2009-227936